I was heavy into r/c buggies way back in late 80's. Several Tamiya kits, recently got rid of my old buggies (a "bigwig" with custom Chevy s10 body kit and chrome rims)
Had to get into the new tech so started with an arrma 1/8 scale brushless Kraton running 6s and molded up, then had to get the shorter wheel base Arrma Outcast, a 1/8 stunt truck. Check out YouTube, double backflips, front flips, and even standing backflips from not moving😁. The acceleration on 6s is unbelievable.
The arrma forums are very much like m2/m3 forums. Lots of mods to do, it's endless. Currently I'm tweaking oil weights and spring rates on the shocks.

That was My attempt at embedding a video of my backflips off a softball backstop. I'm not sure how much air but it's a full size backstop. This running 6s on a stock Arrma Outcast. Before I modded it up.

I currently have several RC's. Losi XXL2-e, Arrma outcast and Nero, Axial Yeti XL, traxxas 1/16 e-revo vxl, and HPI savage xs and XL.
The XXL2-e took a lot of abuse at concrete skate parks before it broke, but I just picked up the Arrma nero and I'm in process of bulletproofing it

Just saw this thread. I own a hobby store (franchise) and sell the whole Traxxas line. The slash is one of our best sellers. Get the 4X4 VXL (brushless) version if you can.

So I'm trying to keep cost down since it's just something for around the house, and maybe take to the park. My son is only 3-years old, so maybe in a few years, it can become his first one and then I'll upgrade.

So I took a look at the Slash and there's a few options. Again, since I'm trying to keep cost down, 2wd is probably fine for me.

Slash
Slash iD (what is iD?)
Ford Raptor iD (I assume this is a Slash but with a Raptor body?)
Slash OBA (I don't need this)
Slash VXL w/ TSM (I think this is brushless, which is what I probably should get)

So of the above, the last one probably is best, right? And what else do I need? Does it come with a remote, batteries and a charger?

Right now, I'm leaning towards to the Traxxas Ford Raptor. I figure it's pretty good out of the box, and I'd just buy a lipo battery for it.

What's a good lipo battery and charger? And I assume I don't need to modify the Raptor when switching to lipo?

Also really love the way the below truck looks, so I could definitely do the base model Slash also and just make it looks like this:

And I want to keep it affordable. Right now, 2wd brushes is just fine.

That is a good choice. The basic Slash comes in several body styles and colors. The newer Traxxas trucks including the Slash (Raptor is part of the Slash line) accept Lipo batteries (only 2 cell lipos for the brushed version)
You will need to get a Traxxas brand battery and charger (lipo and Nimh charger starts at $59.99) since Traxxas uses "smart plug" proprietary technology. Very easy to use.
Also do not forget that even the basic version of the Slash can be later upgraded to the VXL version (up to 60 mph) by swapping the ESC (electronic speed control) and motor.
Hope it helps

So I was looking at the Traxxas Slash. Seems to be getting good reviews. Does anyone have one?

I built many slashes back in my day and it is a great platform to start off on.

I would suggest going with the 4WD. The extra traction alone is worth it on both pavement and dirt. Less likely you will get stuff in some deep stuff if you go backyard dirting and if you ever do upgrade the motor to brushless...you have a better chance of staying straight instead of flipping sideways.